1. Field of the Invention
The present invention relates to a vertical cavity surface emitting laser (VCSEL) which has a laser light emitting region on the top face, particularly to a VCSEL which can be suitably applied to purposes necessitating light output of the low-order transverse mode.
2. Description of the Related Art
A VCSEL emits light in the direction perpendicular to a substrate differently from the existing edge-emitting laser diodes. In the VCSEL, many devices can be arranged in a two-dimensional array on the same substrate. Therefore, the VCSEL has recently attracted attention as a light source for a digital copying machine or a printer.
In the past, in the foregoing VCSEL, a pair of multilayer film reflectors is formed on the semiconductor substrate, and an active layer becoming a light emitting region is provided between the pair of multilayer film reflectors. In one of the multilayer film reflectors, in order to improve efficiency of current injection into the active layer and lower the threshold value current, a current confinement layer having a structure that a current injection region is confined is provided. Further, an n-side electrode is provided on the bottom face side, and a p-side electrode is provided on the top face side. The p-side electrode is provided with a light emitting window to emit laser light. In the VCSEL, current is confined by the current confinement layer, injected into the active layer where light is emitted. While the emitted light repeats reflection between the pair of multilayer film reflectors, the light is emitted as laser light from the light emitting window of the p-side electrode.
In general, the foregoing VCSEL has disadvantages such as nonuniformity that the polarization direction varies according to variation of devices and instability that the polarization direction is changed depending on output and ambient temperatures. Therefore, when such a VCSEL is applied to an optical device with polarized wave dependence such as a mirror and a beam splitter, for example, when the VCSEL is used as a light source for a digital copying machine and a printer, there is a disadvantage that the variation in the polarization direction causes a difference in an image location of an image and output, leading to blur and irregular color.
Further, it is known that in the foregoing VCSEL, while basic transverse mode oscillation is mainly generated in the central region of the laser light emitting region, high-order transverse mode oscillation is mainly generated in the peripheral region of the laser light emitting region. Therefore, there is a disadvantage that when the light emitting window is excessively widened in order to obtain a high-output VCSEL, laser light of the high-order transverse mode is also outputted at high output.
Therefore, in order to solve the former of the foregoing disadvantages, a plurality of techniques for providing a polarization control function inside the VCSEL to stabilize the polarization direction in one direction have been reported.
For example, as one of such techniques, there is a technique using a special gradient substrate made of gallium arsenic (GaAs) with a normal line of face (311). When a VCSEL device is structured by using such a special gradient substrate, gain profile for direction [−233] become high, and thus the polarization direction of laser light can be controlled in this direction. Further, in this case, the polarization ratio of laser light is greatly high, and therefore this technique is effective for stabilizing the polarization direction of the VCSEL in one direction.
Further, Japanese Unexamined Patent Application Publication No. 2001-525995 discloses a technique for forming a discontinuous portion in part of a metal contact layer which does not affect on characteristics of laser light emitted from a light emitting window, and obtaining polarization in parallel with the boundary of the discontinuous portion.
Further, in order to solve the latter of the foregoing disadvantages, many techniques for controlling transverse mode oscillation have been reported.
For example, Japanese Unexamined Patent Application Publication No. H10-56233 discloses a technique for providing a loss decision device in which when a central portion of a laser light emitting region is a starting point, as the emitting position departs from the starting point, the reflectance loss is gradually increased. Further, Japanese Unexamined Patent Application Publication No. 2000-22271 discloses a technique that a second adjustment layer and a first adjustment layer for decreasing reflectance of a peripheral region surrounding a laser light emitting region are provided in this order on a light emitting face.